A conventional excavating/slewing work truck is provided at a driver's seat thereof with operation levers for rotating parts of a working machine such as a boom, an arm and a bucket. Each of the levers is connected directly or through a pilot valve to a spool of a corresponding change-over valve so as to slide the spool for changing the change-over valve.
As shown in FIG. 14, a change-over valve for a boom cylinder 23 is a three-position pilot change-over valve having six ports. When the corresponding operation lever is rotated from a neutral position to a boom down position, open areas of three oil paths in the change-over valve are gradually changed so as to make a speed-control during movement of the spool from its neutral position to its full stroke position.
The areas of the three oil paths are shown in FIG. 12. When the spool is in the neutral position, a third oil path 43 is opened for free passage so as to connect a first pump port P1 to a tank port T1, while a boom cylinder bottom side port (hereinafter, referred to as “bottom port”) CB, a boom cylinder rod side port (hereinafter, referred to as “rod port”) CR, a second pump port P2 and a second tank port T2 are blocked.
While the spool is moved for lowering the boom from the neutral position, the open area (c) of the third oil path 43 is rapidly restricted at an early period of the movement of spool, then it is gradually restricted, and finally the third oil path 43 is perfectly closed when the spool reaches its full stroke position.
During the same movement of spool, the open area (a) of a first oil path 41 for connecting the bottom port CB to the second tank port T2 and the open area (b) of a second oil path 42 for connecting the second pump port P2 to the rod port CR are gradually opened. The open area (a) of the first oil path 41 is open but restricted to some degree when the spool reaches its full stroke. The open area (b) of the second oil path 42 is rapidly widened just before the spool reaches the full stroke, so as to be larger than the open area (a) of the first oil path 41.
While the boom is lowered, the lowered boom is gravitationally accelerated in an early period between times t1 and t2, as shown in FIG. 13, however, an amount of oil supplied from the second oil path is increased. However, when a time t3 is reached, the increased oil supplied from the second path becomes such a level as to suddenly increase the lowering speed, thereby causing a shock.
Therefore, the first oil path is restricted so as to be narrower than the second oil path.
However, consequently, the pressure on the bottom side of the cylinder is increased so as to increase the pressure on the rod side thereof and the pump pressure, thereby causing a large power loss.
Such a power loss may be reduced by a conventional art disclosed in Japanese Laid Open Gazette No. Hei 10-89317, wherein pressure in a drain oil passage to a tank is detected for adjusting the amount of oil discharged from a pump so that oil discharged from the pump is reduced when a boom is lowered. However, this art requires a complicated hydraulic circuit and an expensive variable displacement hydraulic pump.